While a number of recent efforts are being made to achieve a finer pattern rule in the drive for higher integration densities and operating speeds in LSI devices, DUV and EUV lithography is thought to hold particular promise as the next generation in microfabrication technology. In particular, photolithography using an ArF excimer laser as the light source is thought requisite to the micropatterning technique capable of achieving a feature size of 0.13 μm or less.
The ArF lithography started partial use from the fabrication of 130-nm node devices and became the main lithography since 90-nm node devices. Although lithography using F2 laser (157 nm) was initially thought promising as the next lithography for 45-nm node devices, its development was retarded by several problems. A highlight was suddenly placed on the ArF immersion lithography that introduces a liquid having a higher refractive index than air (e.g., water, ethylene glycol, glycerol) between the projection lens and the wafer, allowing the projection lens to be designed to a numerical aperture (NA) of 1.0 or higher and achieving a higher resolution. See Journal of Photopolymer Science and Technology, Vol. 17, No. 4, p 587 (2004). While the ArF immersion lithography has entered the commercial stage, the technology still needs a resist material which is substantially insoluble in water.
In the ArF lithography (193 nm), a high sensitivity resist material capable of achieving a high resolution at a small dose of exposure is needed to prevent the degradation of precise and expensive optical system materials. Among several measures for providing high sensitivity resist material, the most common is to select each component which is highly transparent at the wavelength of 193 nm. For example, polyacrylic acid and derivatives thereof, norbornene-maleic anhydride alternating copolymers, polynorbornene, ring-opening metathesis polymerization (ROMP) polymers, and hydrogenated ROMP polymers have been proposed as the base resin. This choice is effective to some extent in that the transparency of a resin alone is increased.
Studies have also been made on photoacid generators (PAGs). Sulfonium salts comprising a triphenylsulfonium cation and a perfluoroalkanesulfonic acid anion are generally used as the PAG in ArF chemically amplified resist compositions. These PAGs generate perfluoroalkanesulfonic acids, especially perfluorooctanesulfonic acid (PFOS), which is difficult to apply to resist materials because of its risks of difficult degradation, biological concentration and toxicity. Although photoacid generators capable of generating perfluorobutanesulfonic acid are used in the existing resist materials, it is still difficult to achieve a high resolution because the acid generated therefrom are highly diffusible. To solve the problem, partially fluorine-substituted alkanesulfonic acids and salts thereof were developed. Among others, a PAG having 2-acyloxy-1,1,3,3,3-pentafluoropropane-1-sulfonic acid was developed because of ease of incorporation of a substituent group for controlling solubility in resist solvent, stability, and diffusibility (JP-A 2007-145797). This patent document describes, in the prior art section, PAGs capable of generating α,α-difluoroalkylsulfonic acids upon exposure, for example, di(4-tert-butylphenyl)iodonium 1,1-difluoro-2-(1-naphthyl)ethanesulfonate, and PAGs capable of generating α,α,β,β-tetrafluoroalkanesulfonic acids. Although these PAGs have a reduced degree of fluorine substitution, they suffer from several problems that they are insufficiently labile because of the absence of a labile substituent group such as ester structure, the molecular design for changing the size of alkylsulfonic acid is limited, and the starting fluorinated reactants are expensive.
As to the cation, triphenylsulfonium cations are widely used. However, they have the drawback that they exhibit substantial absorption at the ArF exposure wavelength (193 nm) to reduce the transmittance of a resist film, sometimes leading to low resolution. Aiming at a higher sensitivity and resolution, JP 3632410 reports the development of 4-alkoxy-1-naphthyltetrahydrothiophenium cations and JP 3995575 discloses a resist composition comprising a resin having a plurality of acid labile groups in combination with such salt. The 4-alkoxy-1-naphthyl-tetrahydrothiophenium salt suffers from a low stability in resist solution due to the alkylsulfonium salt structure susceptible to nucleophilic displacement reaction and a substantial difference in line width or pattern profile between grouped and isolated patterns (collectively referred to as “dark/bright bias,” hereinafter). In particular, the pattern profile difference between dark and bright areas is a problem. The dark area is a light-shielded area including a line-and-space pattern flanked with bulk patterns (in the case of positive tone resist), and the bright area is a transmissive area including a 10 line-and-space pattern flanked with broad spaces (in the case of positive tone resist). Although optical conditions at the center of the 10-line-and-space pattern are equal between the dark and bright areas, a pattern profile difference arises between the dark and bright areas.